It is common to pump fluids that contain particulates into oil and gas wells. For example, fracturing fluids typically contain proppant particles, such as sand or small ceramic or glass beads, that typically range in size from U.S. Standard Sieve sizes 60 through 16 (0.01 to 0.05 inches, 0.025 to 0.12 cm), and occasionally from U.S. Standard Sieve sizes 100 through 10 (0.006 to 0.079 inches, 0.015 to 0.20 cm). Other fluids containing particles are used for abrasive jetting in oil or gas wells. Slurries (mixtures of liquids and solid particles) are more difficult to pump than particle-free fluids. The presence of solid particles adversely affects pump efficiencies and valve lifetimes, especially at high pressures and/or high flow rates.
Reciprocating plunger pumps are frequently used by oil field service companies to pump proppant-containing fracturing fluids into oil and gas formations. These pumps typically include valve assemblies that are biased toward the closed position. When the motion of the plunger creates fluid flow resulting in a differential pressure across the valve, the differential pressure forces the valve open. However, when the forward motion of the plunger slows and the valve begins to close, solid particles in the fluid can become trapped within the valve assembly. The trapped solids prevent the valve from fully closing and thereby reduce the efficiency of the pump. Trapped solids can also damage the valve assembly components and reduce the useful life of the valve assembly.
The valve assemblies in reciprocating plunger pumps typically contain an area where the metal surface of the valve closure member contacts the metal surface of the valve seat member when the valve is closed. That area is commonly referred to, and is defined herein, as the “strike face area.” There is little or no damage done to the metal surfaces of the valve components when only clear fluids (e.g., clear liquids, such as water or gelled aqueous fluids) are pumped through the valve assembly. The valve lifetime can be quite long and may even outlast the fluid end of the pump in endurance test runs when the pumping medium is a clear fluid. However, when the pumping medium is a slurry, such as a fracturing fluid with proppant, the metal contact surfaces in the strike face area are severely damaged by erosion, abrasion and by pitting caused by solid particles in the fluid. If solid particles are trapped between the metal surfaces as the valve closes, the closing force of the valve is applied to the metal surfaces through the particles rather than being spread uniformly across the strike face area. The localized contact forces, Hertzian contact forces, at the interface of the trapped particles and the metal surface cause pitting in the metal surface. The damage caused by trapped particles is extensive. The valve life can be less than an hour under extreme conditions. Attempts to mitigate the damage to the valve assemblies have been made. One such technique involved an attempt to minimize or replace the metal-to-metal contact in the strike face area by including a resilient elastomeric insert in the closure member or the valve seat. While useful, this technique has not been wholly successful. Solid particles are still trapped at the outer perimeter where the resilient insert forms a hydraulic seal as it closes against the metal surface of the valve seat member. Damage to the metal surfaces near to and along that perimeter increases the extrusion gap size that the resilient insert has to span in order to form an effective hydraulic seal.
The mechanisms by which pitting and other valve damage occurs have been addressed in various patents and publications. For example, U.S. Pat. No. (“USP”) 6,701,955 B2, “Valve Apparatus” by McIntire et al., describes how solid particles in a pumped slurry can become trapped between the two metal contact surfaces in the strike face area. The particles tend to be concentrated in specific locations rather than randomly distributed across those surfaces when concentrated slurries of particles are pumped. This creates concentrated stress forces at these locations and leads to localized pitting. The resulting pits or indentations in the metal surfaces are much wider than single particles. Once such pitting has occurred, the pits act as collection points and solid particles tend to concentrate at these locations on subsequent plunger strokes. This greatly accelerates the damage at these locations.
The above-mentioned technique involving resilient inserts has also been addressed by the present inventor. Valves used for slurry service typically have a resilient sealing insert around the outer perimeter of the valve closure member to provide effective valve sealing. Pressure applied to a closed valve forces the resilient sealing insert to become a hydraulic seal and a portion of the insert is extruded into the gap between the valve closure member and the valve seat member. For the insert to affect a hydraulic seal upon valve closure, the insert must protrude from the valve closure member toward the valve seat member when the valve is open. The amount of protrusion of the insert is called the insert offset. When the valve is nearly closed, the resilient sealing insert contacts the valve seat member before the contact surfaces of the valve closure member and the valve seat member make contact. When the valve is closed, the resilient sealing insert is deformed against the seat member to form the hydraulic seal, and metal-to-metal contact occurs between the valve closure member and the valve seat member in the strike face area. The resilient insert material does not compress but deforms. Repeated deformation of the insert material causes internal heat build-up and material stress within the insert material, and this can damage it. The insert material has low thermal conductivity, and even when bathed in flowing fluid the insert can overheat and be permanently deformed if exposed to large percentage deformations of the insert material.
Damage to the valve insert is also caused by large deformations of the insert material beyond its elastic limit. The elastic limit is an intrinsic property of the material, so the critical deformation is the percentage deformation defined as the deformation per volume unit of the material. If a large insert deformation occurs over a large volume of the insert material, then the percentage deformation can be low, causing minimal damage.
Proppant trapped under the resilient sealing insert can become temporarily or permanently embedded in the resilient insert material, so that the insert can contact the valve seat and affect a hydraulic seal in the presence of proppant. In the presence of proppant, the metal surfaces of the valve closure member and valve seat member do not form a good hydraulic seal. Under pressures typical of oilfield operations, the resilient insert deforms to press against the outer perimeter of the metal-to-metal contact area and makes the hydraulic seal there.
If proppant is trapped between the contact surfaces of the valve seat member and valve closure member, the metal-to-metal seal is not made. The resilient insert is extruded into the gap between the contact surfaces by the differential pressure across the valve. That differential pressure across the valve apparatus will build from zero, before the valve closes, to the full pump output pressure as the valve closes and the plunger actions continue. When the pressure forces on the valve closure member become high enough to crush proppant trapped between the metal contact surfaces, the gap between the contact surfaces decreases from the proppant diameter to the height of the crushed proppant particles. Just before the proppant is crushed, the insert is subjected to extrusion into a gap width defined by the proppant particles' diameter, with an extrusion pressure just less than the pressure required to crush the proppant. If proppant particles are piled up in the contact area, the extrusion gap can be larger than the diameter of individual particles. After the proppant is crushed, the gap between the two contact surfaces is reduced to the width of the crushed proppant debris. Then the insert is subjected to extrusion into that smaller gap, with an extrusion pressure equal to the maximum differential pressure across the pump.
The resilient sealing insert contacts the valve seat member before the valve closure member contacts the valve seat member. The gap between the sealing insert and the seat of an open valve is smaller than the gap between the valve closure member and the valve seat. This is required in order to have the resilient sealing insert contact the valve seat before the valve closure member and make a hydraulic seal. As the valve closes, the gap between the sealing insert and the valve seat member becomes too small to pass particles in the fluid, while the gap between the valve closure member and the valve seat member is still large enough to pass particles into the region between them. Thus, a standard valve-sealing insert can act as a forward-screening element that concentrates proppant particles in the strike face area, particularly in the critical area near the outer perimeter of the strike face. Such concentrations of proppant particles enhance damage to the contacting surfaces of the valve closure member and the valve seat member.
If the pump is operated in such a way as to have valve lag, i.e. the discharge valve does not close until after the plunger starts its suction stroke, there will be reverse flow through the valve before it closes. Before the valve closes, the insert will approach the valve seat so that the gap between them is less than the proppant diameter. The sealing insert will screen out proppant particles from the reverse fluid flow, preventing the particles from entering the region between the valve closure member and the valve seat member. However, the volume of fluid without proppant, which flows through current valves during the short time interval between the onset of such reverse particle screening and the closure of the valve, typically is insufficient to displace the proppant-laden fluid from the valve before closure. Particles are still trapped between the valve closure member and the valve seat member. Additional fluid without proppant would be required to flush the gap between the contact surfaces of the valve closure member and the valve seat member before the valve closes enough to trap proppant particles in that gap.
The resilient insert should extend down below the frustoconical contact surface of the valve closure member by a distance greater than the diameter of the solid particles in the slurry being pumped. Otherwise, the valve can be held open by solid particles caught between the metal contact surfaces of the valve seat member and valve closure member, without the resilient insert member reaching the valve seat member to affect a hydraulic seal. The extension of the insert member below a parallel extension of the frustoconical contact surface of the valve closure member is referred to as the valve insert member's offset. Current valve assemblies have insert member offsets typically of 0.06 to 0.08 inches. Larger offsets would result in larger insert material deformations leading to heating and material failure. Current valve assemblies were developed for pumping slurries with proppant particles that typically would pass through a U.S. Standard Sieve of 20 mesh. The maximum proppant particle diameter to pass through that mesh is 0.032 inches, so an insert offset of 0.06 inches will allow the valve insert to contact the valve seat while there is proppant between the contact surfaces of the valve body and the valve seat. The insert will be deformed over particles trapped under the insert. However, larger proppant particles are being used today to increase the efficiency of oil and gas withdrawal following fracturing operations. Proppants pumped today can include some particles with diameters larger than the typical insert offsets of current valve assemblies.
Increasing the offset of the resilient insert member to accommodate larger diameter proppant particles, by allowing the insert member to contact the valve seat member while there are proppant particles between the contact surfaces of the valve closure and valve seat members, increases the deformation of the insert member when the valve is closed. That increases heating and deformation damage to the insert member. Additional deformation damage to the resilient insert member is caused by trapping proppant particles between the resilient insert member and the valve seat member when the valve is closed. Proppant particles trapped between the resilient insert member and the valve seat member deform the resilient insert member and may be embedded in the insert. Larger proppant particles will cause significantly increased deformation damage and embedment damage when trapped between the resilient insert member and the valve seat member when the valve closes.
U.S. Pat. No. 6,701,955 B2, “Valve Apparatus” by McIntire et al., describes the problems of packing proppant particles between the frustoconical contact surfaces of the valve closure member and the valve seat member, particularly near the outer perimeter of the strike face area, and teaches some ways to flush the proppant out of that space, mainly by trapping proppant particles between the resilient insert and the valve seat. The present invention has advantages over the apparatus described in U.S. Pat. No. 6,701,955 in that: a) it provides a volume of trapped slurry from which proppant is screened as that slurry is pumped through the area between the contact surfaces, b) it provides a cavity to accommodate proppant particles trapped under the insert without deforming and damaging the insert, and c) it provides an insert that will seal in the presence of large proppant particles without requiring large percentage deformation of the insert material.
U.S. Pat. No. 2,495,880 by Volpin shows a cylindrical plug, as part of the valve closure member, that protrudes down into the throat of the valve seat member when the valve is closed. U.S. Pat. No. 6,701,955 B2, “Valve Apparatus” by McIntire et al., teaches the use of such cylindrical plugs to increase the speed at which the valve closure member rises when the plunger starts to move forward and pump fluid through the valve apparatus, and to retard the descent of the valve closure member at the end of the plunger forward stroke. Retarding the descent of the valve closure member promotes valve lag that reduces the amount of proppant particles trapped under the valve and makes the reverse pumping aspect of the current invention more effective.
U.S. Pat. No. 7,000,632 B2, “Valve Apparatus” by McIntire et al., teaches the use of protrusions around the outer perimeter of the contacting surface of the resilient insert to provide a screening gap between that surface and the valve seat. This allows clear fluid (i.e., fluid without proppant particles) to flow in a reverse direction, from downstream of the valve, through the valve and to flush proppant particles from the gap between contact surfaces of the valve closure member and the valve seat member before the valve closes.
Another problem with conventional valves for high-pressure slurry pumps, such as the reciprocating plunger pumps mentioned above, is the impact of the valve closure member on the valve seat member when the valve exhibits valve lag, closing after the pump plunger has reversed direction. Valve lag can be useful for slurry pumps, because it can reduce the number of particles concentrated in the valve before closure. However, large amounts of valve lag lead to damage of conventional valves, as the valve closure member slams into the valve seat member with high velocity and considerable force in closing.
There is a need for improved valve assemblies that reduce the incidence of damage to the valve closure member and valve seat member caused by particulates in slurries. There is also a need to reduce valve insert damage due to compressive deformation, particularly for inserts with offsets large enough to accommodate large particles. There is also a need for valve assemblies that can operate efficiently while pumping slurries with large proppant particles. These needs are addressed by the present invention.
A valve apparatus that closes without particles trapped between the two metal contact surfaces in the strike face area would permit one to pump slurries without valve damage. Valve damage could also be significantly diminished by reducing or eliminating particles trapped near the outer perimeter of the metal contact surfaces. These are some of the objects of the present invention.